Newswise — In the future, the utilization of atomic timepieces might enable scientists to investigate the underlying powers of the cosmos. As a constituent of a global partnership, researchers from LMU have achieved a pivotal breakthrough in this domain.
Atomic clocks achieve such remarkable precision that their time deviation is less than a second over a span of 30 billion years. However, the potential for even greater time measurement accuracy lies within nuclear clocks. Furthermore, these advanced timepieces would empower scientists to explore fundamental physical phenomena on a more profound level. "We are referring to the forces that bind the essence of our world together," explains Professor Peter Thirolf, a physicist from LMU who has dedicated many years to researching nuclear clocks. Unlike traditional atomic clocks, this innovative clock variant would detect forces within the atomic nucleus, unveiling a vast array of research domains that were previously inaccessible. Dr. Sandro Kraemer, a collaborator of Thirolf and a driving force behind the project during his doctoral studies at KU Leuven in Belgium, emphasizes the groundbreaking potential of this endeavor.
Thirolf and Kraemer, prominent figures in the pursuit of nuclear time, have established themselves as frontrunners. Operating from the Chair of Experimental Physics in Garching, these two scientists have achieved a significant breakthrough in their quest for the first-ever nuclear clock, collaborating with an international team. Their remarkable findings, published in the esteemed journal Nature, unveil a new experimental methodology enabling them to accurately characterize the excitation energy of thorium-229. This specific atomic nucleus is anticipated to serve as the fundamental component for future nuclear clocks. Acquiring precise knowledge of the required excitation frequency is vital to determine the feasibility of this groundbreaking technology.
The innermost clock
To establish a functional clock, two key components are required: a periodic oscillation source and a mechanism for tallying the oscillations. In the case of a grandfather clock, a mechanical pendulum fulfills the role of the oscillation source, with its swings recorded by the clock's internal workings. Conversely, atomic clocks rely on the atomic structure to serve as the timekeeping element. Specifically, electrons within the atomic shell are excited, transitioning between high and low energy levels. The crucial task then becomes counting the frequency of light particles emitted by the atom as the excited electrons return to their ground state. By monitoring this emission, atomic clocks accurately measure time.
The fundamental principle behind nuclear clocks is indeed quite similar. However, in this instance, the focus shifts to the atom's nucleus, which harbors a range of energy states. By effectively exciting these states through precise laser stimulation and subsequently detecting the radiation emitted by the nucleus as it returns to its ground state, a nuclear clock can be realized. The challenge lies in identifying a suitable atomic nucleus for this purpose among the numerous nuclei known to science. Remarkably, only one nucleus, thorium-229, has emerged as a potential candidate for nuclear timekeeping. It is important to note that even the feasibility of utilizing thorium-229 in this manner remained purely theoretical for a considerable period of time.
A nucleus like no other
The exceptional characteristic of thorium-229 lies in its ability to be excited to a high-energy state using a relatively low-frequency light, which can be achieved with UV lasers. However, progress in research on this topic stagnated for four decades as scientists possessed a theoretical understanding that an atomic nucleus possessing the desired properties might exist but lacked experimental confirmation. In 2016, Professor Thirolf's research team at LMU achieved a groundbreaking milestone by directly verifying the excited state of thorium-229's nucleus. This breakthrough ignited the commencement of the nuclear clock race. Subsequently, numerous research groups worldwide have actively joined the pursuit, contributing to the advancement of this captivating field.
To establish a functional clock, precise synchronization between the timekeeping element and the clockwork is essential. In the context of a nuclear clock, this synchronization necessitates knowledge of the precise frequency at which the atomic nucleus of thorium-229 oscillates. Once this frequency is determined, lasers can be developed to precisely excite the nucleus at that specific frequency. Dr. Kraemer aptly illustrates this concept using the analogy of a tuning fork. Just as a musical instrument strives to match the frequency of a tuning fork, the laser aims to hit the exact frequency of the thorium nucleus. This meticulous alignment ensures the accuracy and reliability of the nuclear clock.
Exploring all possible frequencies using different lasers would indeed be an arduous and time-consuming process, particularly considering the need to develop lasers within the appropriate UV light spectrum. In order to narrow down the range of frequencies at which thorium-229's oscillation occurs, the researchers adopted a different approach. "Nature, at times, grants us various pathways," remarks Thirolf. Interestingly, lasers are not the sole method of inducing the excited state in the thorium nucleus. The same state can also be achieved when radioactive nuclei decay and transform into thorium-229. Thus, the researchers shift their focus to the predecessors of thorium-229, such as its grandparents and great-grandparents, to initiate the desired state. This alternative method allows them to explore and gain insights into the frequency range required for the nuclear clock without solely relying on laser experimentation.
ISOLDE is forging new paths
The ancestors in question, namely francium-229 and radium-229, are not naturally occurring elements and must be artificially synthesized. Currently, only a handful of facilities worldwide possess the capability to produce them. One such facility is the ISOLDE laboratory located at the European Organization for Nuclear Research (CERN) in Geneva. Remarkably, this laboratory has fulfilled the long-standing aspiration of alchemists by enabling the transformation of one element into another. The process involves bombarding uranium nuclei with highly accelerated protons, resulting in the creation of various new nuclei, including francium and radium. These synthesized elements then undergo rapid decay, ultimately leading to the formation of the radioactive precursor nucleus of thorium-229: actinium-229.
To capture the decay of actinium-229 into thorium-229 in its excited state, Kraemer, Thirolf, and their international collaborators ingeniously embed the intricately synthesized actinium within specialized crystals. Within these crystals, the actinium undergoes decay, transitioning into thorium while remaining in an excited state. As the thorium nucleus returns to its ground state, it emits the crucial light particles whose frequency is pivotal for the development of the nuclear clock. However, successfully demonstrating this phenomenon is a formidable challenge. Kraemer emphasizes the need for precise placement of the nuclei within the crystal, as any deviation would result in the absorption of energy by the surrounding electrons. Consequently, none of the measurable particles would escape, hindering the observation and measurement of the emitted light particles.
Earlier endeavors that involved inserting uranium into the crystal lattice, as a substitute for actinium, faced significant challenges. Professor Thirolf clarifies that when uranium-233 decays into thorium-229, a recoil effect occurs, leading to disruptive consequences within the crystal structure. This recoil effect impairs the stability of the crystal, hindering the desired measurements. In contrast, the decay of actinium into thorium induces significantly less damage to the crystal lattice. This crucial difference influenced the researchers' decision to embark on the painstaking path of using actinium in their collaborative study with CERN, as it offered a more favorable environment for their experimental objectives.
The diligent efforts and patience of the team have proven fruitful, as their new methodology enabled them to accurately determine the energy associated with the state transition. Moreover, they successfully demonstrated the feasibility of a nuclear clock built around thorium embedded within a crystal. These solid-state-based clocks possess a notable advantage over other approaches: they can deliver measurement results more rapidly due to their ability to work with a larger number of atomic nuclei. This achievement opens up promising possibilities for advancing the field of precision timekeeping and brings us closer to the realization of practical nuclear clocks.
A matter of time
According to Professor Thirolf, the team has now obtained valuable information about the approximate wavelength required for the nuclear clock. Building upon these insights, their next task is to progressively refine and narrow down the exact transition energy. Initially, they will employ a laser for excitation, and subsequently, they can utilize increasingly precise lasers to further refine the frequency with higher accuracy. To expedite this process, the researchers utilize a remarkable tool called a "frequency comb," developed by Professor Theodor Hänsch, Thirolf's colleague at LMU and recipient of the Nobel Prize in Physics in 2005 for this innovation. The frequency comb acts as a metaphorical rake, allowing scientists to simultaneously scan hundreds of thousands of wavelengths until they identify the precise frequency they seek. This approach significantly accelerates the search process, ensuring that the endeavor progresses efficiently.
Despite the challenges that lie ahead, including a deeper understanding of the thorium isomer, laser development, and theoretical advancements, the researchers, such as Professor Thirolf, believe that persevering on the path towards nuclear clocks is well worthwhile. The project holds immense potential for a wide range of applications, both in fundamental physics research and practical use cases. By employing a nuclear clock, scientists could detect and monitor minuscule changes in the Earth's gravitational field, such as those occurring during tectonic plate movements or prior to volcanic eruptions. With the recent progress achieved, the realization of nuclear clock prototypes seems within close reach, possibly emerging within the next decade. The two physicists express hope that these prototypes may even be ready in time for the planned redefinition of the second in 2030, which aims to establish a more precise standard definition for the second using cutting-edge atomic clocks, and potentially even the first nuclear clocks. The future holds promising prospects for these innovative timekeeping devices.
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